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A turf war is developing between rootkit-touting cybercrooks over control of infected PCs.

Rootkits are strains of malware designed to hide below the level of anti-virus scanners and programmed to carry out functions such as click fraud. The Russian developer of one of the more potent strains of rootkit, TDL, is supplementing his income by selling the source code for the malware to other cybercrooks via underground forums.

But one of the groups who bought the code has done its own tinkering to develop a related strain of rootkit, called ZeroAccess. As well as adding click fraud modules the second group has begun bundling functionality that uninstalls the TDL rootkit from infected machines, effectively double crossing the original TDL3 author, according to an analysis by web security firm Webroot.

"The original author of the TDL3 rootkit made two versions of TDL3. He kept the second version of the rootkit code for himself and sold the first version to the guys behind ZeroAccess," Jacques Erasmus of Webroot told El Reg.

The group who bought the TDL3 code from the original author added a module called z00clicker that allowed infected machines to be used in click fraud around 12 months ago. ZeroAccess uses an ad clicker plugin called z00clicker2. In addition, the filesystem used by both TDL and ZeroAccess are similar, another sign that the adapted version of TDL3 and ZeroAccess are related.

Adapting purchase source code is all part of software development, whether in the legitimate or underground cyber-economies. However Webroot's analysis suggests that things have turned sour between the original TDL3 and ZeroAccess groups because ZeroAccess has begun bundling functionality to remove TDL3 from infected machines with a specific module called Anti-TDL.

"TDL3 Authors sold a version of TDL3 sourcecode to ZeroAccess authors. Now ZeroAccess guys are double crossing the TDL3 author by uninstalling the TDL rootkit," Erasmus told El Reg, adding that the behaviour recalls earlier turf wars between purveyors of the ZeuS and SpyEye banking Trojans when the latter begun bundling functions to nuke instances of ZeuS from PCs it infected in order to gain sole control of compromised boxes.

ZeroAccess is spreading rapidly via cracks and keygens websites. The malware is particularly difficult to eradicate because it goes out of its way to prevent security software from running on infected machines, as illustrated by a video on YouTube made by Webroot. ®